The gain-of-function BRAFT1799A mutation is highly prevalent in melanomas and in thyroid cancers. In the latter, BRAF mutations confer poor prognosis. The spectacular early results of clinical trials in patients with metastatic melanoma treated with the selective RAF inhibitor PLX4032 serve as yet another proof that advanced cancers often retain dependency on oncogenic kinases activated in early stages of tumorigenesis. However, response to selective kinase inhibition is not uniform between tumors of different lineages harboring the same mutation (e.g. in BRAF). The mechanisms that account for this are unknown. Data generated during this granting cycle point to important differences in the biology of thyroid cancers initiated by oncogenic BRAF as compared to melanomas, as well as in the response of murine thyroid cancers and human cell lines to MEK and RAF inhibitors. The experiments proposed in this application aim to explore the key determinants of this differential response, and whether thyroid cancers can be sensitized to MAPK inhibition by selectively targeting feedback events that arise after perturbation of the network. For this we will: 1. Examine mechanisms of primary resistance to RAF and MEK inhibitors, and of feedback reactivation of RTK signaling in thyroid cancer cell lines. 2. Identify the profile of RTK activation after acute exposure to MEK or RAF inhibitors in vivo, and determine the effects of suppressing their activity. 3. Explore mechanisms of acquired resistance to the selective RAF inhibitors in BRAF (+) thyroid cancer cell lines, murine thyroid cancers, and in metastatic lesions of BRAF (+) thyroid cancer patients treated with PLX4032. PUBLIC HEALTH RELEVANCE: This proposal will investigate why thyroid cancers appear to be comparatively resistant to targeted drugs that block the activity of the cancer protein encoded by the BRAF oncogene, which is a common cause of the disease. We have discovered that after thyroid cancer cells or mice with thyroid cancer are treated with BRAF antagonists, they respond by turning on the activity of certain growth factor receptors, which allows them to continue to grow. By exploring how these cancers become resistant to the RAF inhibitors, we believe we can block the resistance mechanisms and obtain better responses to these promising targeted therapies.